The present invention relates generally to semiconductor devices and more particularly to high-k dielectrics.
There is a long felt need for small, portable personal devices. These devices include cellular phones, personal computing equipment, and personal sound systems, which are sought in continuously smaller sizes and with continuously lower power requirements. While smaller and more portable devices are sought, computational power and on-chip memory requirements are increasing. To meet these requirements, small, densely packed semiconductor device features, such as features forming field effect transistors (FETs), are needed.
FETs include a source and drain region separated by a channel. A control gate, typically of polysilicon, is formed over the channel and is electrically separated from the channel by a gate dielectric layer, which is typically silicon dioxide. At a given potential difference, a current will either flow or not flow across the channel between the source and the drain depending on the voltage applied to the control gate.
A limitation on the degree to which FETs can be scaled down relates to the gate dielectric layer. As FETs become progressively smaller, the capacitance between the control gate and the channel must be proportionally increased. To achieve this capacitance, conventional silicon dioxide gate dielectric layers must be very thin: about 5 nm or less for devices with 0.25 xcexcm features. Issues with gate dielectric layer manufacturing and performance arise at such small thicknesses. Providing a gate oxide layer with sufficient uniformity becomes difficult and leakage current due to quantum mechanical tunneling through the gate layer becomes significant.
In view of these issues, it has been proposed to replace silicon dioxide gate dielectrics with so called high-k dielectrics. A high-k dielectric layer can be made thicker than an electrically equivalent layer of silicon dioxide while providing a given degree of capacitance. High-k dielectrics typically have a dielectric constant of at least about 5, whereby the gate dielectric layer is at least about five times thicker than an electrically equivalent silicon dioxide gate dielectric layer.
Use of high-k dielectrics presents another host of issues. Obtaining a high-k dielectric layer that adheres well to a silicon substrate can be difficult as can controlling the thickness of the high-k dielectric layer. Polysilicon gates are generally doped with boron and many high-k dielectrics are ineffective in preventing diffusion of boron from the gate to the channel region, where the boron can have a variety of undesirable effects. In view of these issues, there is a continuing demand for high-k dielectrics and processes for forming them.
The following presents a simplified summary of the invention in order to provide a basic understanding of some of its aspects. This summary is not an extensive overview of the invention and is intended neither to identify key or critical elements of the invention nor to delineate its scope. The primary purpose of this summary is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
One aspect of the invention relates to forming a high-k dielectric layer. The high-k dielectric comprises a Group IVB metal compound, especially HfO2, HfSixOy, or HfSixOyNz. According to the invention, these compounds are formed by molecular layer deposition. The Group IVB metal compound can be formed immediately over a silicon channel region.
According to another aspect of the invention, molecular layer deposition is used to add silicon oxynitride to the dielectric layer. The silicon oxynitride provides a barrier to diffusion of dopants from the gate to the channel region. The silicon oxynitride is formed by successively depositing atomic layers of oxygen, silicon, and nitrogen.
Other advantages and novel features of the invention will become apparent from the following detailed description of the invention and the accompanying drawings. The detailed description of the invention and drawings provide exemplary embodiments of the invention. These exemplary embodiments are indicative of but a few of the various ways in which the principles of the invention can be employed.